Template:If empty
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This template is used inside other templates. It takes any number of parameters and returns the first found to be defined and non-empty. If none is found, nothing is returned. By writing {{if empty|a|b|c}} instead of {{#if:a|a|{{#if:b|b|c}}}}, expression a and b do not have to be repeated and evaluated twice. The template provides a fallback order, similar to a try catch based "It is easier to ask for forgiveness than permission" programming style.
Typical usage is like this:
{{If empty |{{{logo|}}} |{{{image|}}} |{{{picture|}}} |default.svg}}- ("Return {{{logo|}}} if it has a value; else {{{image|}}} if it has a value; else {{{picture|}}} if it has a value; else return "default.svg".")
This returns the first of the parameters logo, image and picture that is defined and non-empty, otherwise "default.svg".
Rationale
The MediaWiki parameter default function doesn't return the default value for empty parameters. That is, {{{logo|default.svg}}} does not return "default.svg" if the template was called like this: {{template|logo=}}.
The usual workaround for a single parameter is:
{{#if:{{{logo|}}} |{{{logo}}} |default.svg}}- ("If {{{logo}}} has a value, return it, else return "default.svg".")
But this becomes complex when several parameters are to be checked:
{{#if:{{{logo|}}} |{{{logo}}} |{{#if:{{{image|}}} |{{{image}}} |{{#if:{{{picture|}}} |{{{picture}}} |default.svg}} }} }}- ("If {{{logo}}} has a value, return it; else if {{{image}}} has a value, return that; else if {{{picture}}} has a value, return that; else return "default.svg".")
In these cases, {{if empty}} produces the simpler syntax (as above):
{{if empty |{{{logo|}}} |{{{image|}}} |{{{picture|}}} |default.svg}}
Piping
Parameters used with {{if empty}} must be piped – i.e. include the vertical bar (pipe) symbol (" | ") as a trailing character – so that empty or undefined parameters aren't treated as text and returned incorrectly. Hence, for example, {{{logo|}}}, {{{image|}}} and {{{picture|}}} rather than |logo=, |image= and |picture= in the above examples.
Examples
{{if empty}}-> Returns an empty string.{{if empty|one}}-> one Returns the first parameter that is defined and not empty.{{if empty|one|two}}> one{{if empty|one|two|three|four}}-> one{{if empty|two}}-? two The first parameter is empty/undefined, so is passed over.{{if empty|two|three|four}}-> two{{if empty|two|four}}-> two |{{|if empty|}}-> The only parameter is empty or undefined, so returns an empty string.{{if empty|||||}}-> Returns an empty string.{{if empty|{{{1|}}}|{{{2|}}}|three}}-> three{{if empty|{{{1}}}|{{{2}}}|three}}-> {{{1}}} (No pipe-characters following the names of the first two parameters ("1" and "2"), so the first of these returned as text ("{{{1}}}")).{{if empty|{{{logo|}}}|two}}-> two{{if empty|{{{logo}}}|two}}-> {{{logo}}} (No pipe-character following the parameter name "logo", so the text "{{{logo}}}" returned.){{if empty|p=q}}-> (The template identifies the parameters it receives as parameters 1 to 9, not using names such as "p", etc.)